Characterization of a Dual-Mode Scramjet via Stereoscopic Particle Image Velocimetry

Scramjet propulsion devices have the potential for revolutionizing high-speed atmospheric flight in the future. This includes aerospace planes for space access and high-speed long distance strike vehicles for national defense. Despite over 60 years of research, the high-speed, turbulent, combusting flow field of the engine is not fully understood. In particular, the dual-mode scramjet (DMSJ) is complicated by the ability to operate in different modes of combustion. These modes of combustion are referred to as the ramjet mode of operation, characterized by subsonic combustor inflow, and the scramjet mode of operation, characterized by supersonic combustor inflow. The gap in knowledge stems from the inability to experimentally measure pertinent flow properties and the resulting absence of accurate computational tools. Renewed interest in hypersonic flight and the advancement of laser-based optical measurement techniques provide an opportunity to quantify flow properties that were once not accessible. This includes instream Velocimetry in the supersonic, turbulent, combusting flow field of a DMSJ. Such measurements can provide a fundamental understanding of the convective transport processes in these engines. Of particular interest is how these processes change for the DMSJ when operating in the supersonic, subsonic, and mixed modes of combustion.

This work employs the experimental technique of Stereoscopic Particle Image Velocimetry (SPIV) to measure the 3-dimensional, instantaneous velocity within a DMSJ model. This work is part of the greater collaborative efforts of the National Center for Hypersonic Combined Cycle Propulsion (NCHCCP). Significant design of new DMSJ model hardware was necessary to allow the extensive experimental testing with the end goal of building a one-of-a-kind data set for the development and validation of advanced modeling techniques.

The work presented herein contributes to this goal by measuring the flow field within the combustor section in both the ramjet mode and scramjet mode of operation at four axial locations. These measurements represent the first reported SPIV measurements in the two different modes of operation of a DMSJ and reveal the key similarities and differences between the modes. The influence on the velocity field as the combustion process develops axially is also identified for each mode. Additionally, the measurements serve to quantify the velocity bias associated with the single-stream seeding method. The primary properties presented in the current study are mean velocity magnitude, cross-plane velocity vectors, vorticity, and turbulent kinetic energy. The defining flow features of the scramjet mode of operation are a low-momentum separation region near the fuel injection wall surrounded by a high-speed freestream. Alternatively, the ramjet mode of operation consists of a high-speed fuel jet core surrounded by a low-speed freestream. This data helps to identify the location, size, and intensity of the classic ramp fuel injector induced vortices which act to mix the fuel and air. The results indicate that the vortices expand with the axial location in the flowpath while the intensity decreases. In addition, the peak levels of turbulent kinetic energy have been identified ranging between 10,000-15,000 m2/s2 and are located corresponding to a shear layer present in the axial velocity component. The comprehensive data set provides converged turbulence statistics and the means to quantitatively compare experiments to numerical models. Comparisons with a hybrid LES/RANS computational model (conducted at N.C. State) reveal some areas of improvement to the computational model but overall excellent agreement was observed.